Observer-based cancellation system for implantable hearing instruments

The present invention relates to implanted hearing instruments, and more particularly, to the cancellation of undesired signals from an output of an implanted microphone.

In the class of hearing aid systems generally referred to as implantable hearing instruments, some or all of various hearing augmentation componentry is positioned subcutaneously on, within, or proximate to a patient\'s skull, typically at locations proximate the mastoid process. In this regard, implantable hearing instruments may be generally divided into two sub-classes, namely semi-implantable and fully implantable. In a semi-implantable hearing instrument, one or more components such as a microphone, signal processor, and transmitter may be externally located to receive, process, and inductively transmit an audio signal to implanted components such as a transducer. In a fully implantable hearing instrument, typically all of the components, e.g., the microphone, signal processor, and transducer, are located subcutaneously. In either arrangement, an implantable transducer is utilized to stimulate a component of the patient\'s auditory system (e.g., ossicles and/or the cochlea).

By way of example, one type of implantable transducer includes an electromechanical transducer having a magnetic coil that drives a vibratory actuator. The actuator is positioned to interface with and stimulate the ossicular chain of the patient via physical engagement. (See e.g., U.S. Pat. No. 5,702,342). In this regard, one or more bones of the ossicular chain are made to mechanically vibrate, which causes the ossicular chain to stimulate the cochlea through its natural input, the so-called oval window.

As may be appreciated, a hearing instrument that proposes to utilize an implanted microphone will require that the microphone be positioned at a location that facilitates the receipt of acoustic signals. For such purposes, an implantable microphone may be positioned (e.g., in a surgical procedure) between a patient\'s skull and skin, for example, at a location rearward and upward of a patient\'s ear (e.g., in the mastoid region).

For a wearer of a hearing instrument including an implanted microphone (e.g., middle ear transducer or cochlear implant stimulation systems), the skin and tissue covering the microphone diaphragm may increase the vibration sensitivity of the instrument to the point where body sounds (e.g., chewing) and the wearer\'s own voice, conveyed via bone conduction, may saturate internal amplifier stages and thus lead to distortion. Also, in systems employing a middle ear stimulation transducer, the system may produce feedback by picking up and amplifying vibration caused by the stimulation transducer.

Certain proposed methods intended to mitigate vibration sensitivity may potentially also have an undesired effect on sensitivity to airborne sound as conducted through the skin. It is therefore desirable to have a means of reducing system response to vibration/noise (e.g., caused by biological sources and/or feedback), without affecting sound sensitivity. It is also desired not to introduce excessive electronic noise during the process of reducing the system response to vibration. These are the goals of the present invention.

In an implantable hearing instrument utilizing an implanted microphone, it is often necessary to differentiate between desirable signals and undesirable signals. Desirable signals are those caused by outside or ambient sound, which causes tissue overlying an implanted microphone diaphragm to move relative to an inertial (non accelerating) microphone implant housing. This movement displaces an implanted microphone diaphragm. Accordingly, the implanted microphone generates an output that is indicative of the ambient sound. Undesirable signals may be those caused by vibration-noise. In this regard, undesired signals may be caused by relative movement between overlying tissue and an implanted implant housing which may result from the movement (e.g., due to acceleration, vibration, noise, etc.) of overlying tissue exerting a force on the implanted microphone diaphragm.

Differentiation between the desirable and undesirable signals may be at least partially achieved by utilizing one or more one-motion sensors to produce a motion signal(s) when an implanted microphone is in motion. Such a sensor may be, without limitation, an acceleration sensor and/or a velocity sensor. In any case, the motion signal is indicative of relative movement of the implanted microphone diaphragm and overlying tissue due to non-ambient sound sources such as motion/acceleration. In turn, this motion signal may be used to yield a microphone output signal that is less vibration sensitive.

That is, the output of the motion sensor (i.e., motion signal) may be processed with an output of the implantable microphone (i.e., microphone signal) to provide an audio signal that is less sensitive to non-ambient sources of vibration/noise than the microphone signal alone. For example, the motion signal may be appropriately scaled, phase shifted and/or frequency-shaped to match a difference in frequency response between the motion signal and the microphone signal. That is, a transfer function is determined between the motion sensor and the microphone. The motion signal may be processed according to the transfer function and the resulting processed signal may then be removed from the microphone signal to yield a net, improved audio signal employable for driving a middle ear transducer, an inner ear transducer and/or a cochlear implant stimulation system.

Stated otherwise, the motion sensor signal is shaped to estimate the microphone signal for common stimuli (e.g., acceleration, vibration). When the processed signal is combined with the microphone signal, this cancels the undesired signals in the microphone signal caused by the common stimuli. However, this also introduces motion sensor (e.g., accelerometer) electrical noise into the system. Further the shaping (e.g., filtering) of the motion sensor signal may amplify the electrical noise of the motion sensor. Accordingly, it may be desirable to cancel undesirable signals from the microphone signal without introducing electrical noise into the system. In such an arrangement the signal delivered to the patient may have a better signal to noise ratios (i.e, SNR) especially in the key intelligence band of 1-4 kHz.

Such cancellation, without introduction of electrical noise, may be performed by using the digital output of the hearing instrument. That is, a digital output signal of an implantable hearing instrument may be utilized to remove undesired signals from a microphone output signal (e.g., input signal of the implantable hearing instrument). As with a motion signal, the output signal of the implantable hearing instrument may be appropriately scaled, phase shifted and/or frequency-shaped according to a transfer function between the output and the microphone signal. In this regard, the transfer function may model the feedback path from an implanted auditory stimulation device to the implanted microphone. Again, the output signal may be processed according to the transfer function and the processed signal may be removed from the microphone signal to yield a net, improved audio signal. In this case however, the signals are all digital so there is no introduction and/or amplification of electrical noise into the system.

In order to scale, frequency-shape and/or phase shift a motion or output signal, a variety of signal processing filtering methods may be utilized. For instance, a filter may be utilized to model the transfer function between the motion sensor and the microphone and/or the output signal and the microphone. Such filters may be operative to scale the magnitude and phase of the signals such that they are made to substantially match the microphone signal for common stimuli. Accordingly, by removing a ‘filtered’ signal from a microphone signal, the effects of, for example, feedback associated with motion caused by operation of an implanted auditory stimulation device may be substantially reduced. Further, in the case of the motion sensor signal, generating a filter operative to manipulate the motion signal to substantially match the microphone signal for feedback caused by operation of the implanted auditory stimulation device (e.g., in response to inserted signal), may also allow for manipulating such a motion signal generated in response to undesired signals such as biological noise. In any case, the combination of a filter for filtering a signal (e.g., the motion signal and/or output signal) and the subsequent removal of that filtered signal from the microphone signal can be termed a cancellation filter. Accordingly, the output of the cancellation filter may be an estimate of the microphone acoustic response (i.e., the output signal with undesired signals removed).

Use of such a cancellation filter works well provided that the modeled transfer function of the system remains fixed. However, it has been determined that the transfer function changes with changes in the operating environment of the implantable hearing device. For instance, changes in skin thickness and/or the tension of the skin overlying the implantable microphone result in changes to the transfer function of the modeled systems. Such changes in skin thickness and/or tension may be a function of posture, biological factors (i.e., hydration) and/or ambient environmental conditions (e.g., heat, altitude, etc.). For instance, posture of the user may have a direct influence on the thickness and/or tension of the tissue overlying an implantable microphone. In cases where the implantable microphone is implanted beneath the skin of a patient\'s skin, turning of the patient\'s head from side to side may increase or decrease the tension and/or change the thickness of the tissue overlying the microphone diaphragm. As a result it is preferable that a cancellation filter be adaptive in order to provide cancellation that changes with changes in the operating environment of the implantable hearing instrument.

Accordingly, provided herein are systems and methods (i.e., utilities) that allow for adjusting the transfer function of a filter that is utilized to filter the digital output of an implanted hearing system such that the resulting filtered signal may be cancelled from an implanted microphone output signal. In this regard, the disclosed utilities utilize an observer that is operative to quickly determine, for example, changes in the operating conditions/environment of an implanted hearing instrument and to generate an output indicative thereof. Such an observer may be a module that is operative to determine one or more intended states of the microphone/motion sensor system. This output may then be utilized by the utility to adjust the digital filter to account for the changed operating conditions. Further, as such utilities utilize the digital output of the hearing instrument to cancel signals from the microphone signal, additional electrical noise is not introduced into the system.

According to a first aspect, a utility is provided for use with an implantable hearing instrument having an implanted microphone. The utility includes identifying a current operating condition of an implantable hearing instrument utilizing an observer. Based on the current operating condition as determined by the observer, filter settings may be established for a cancellation filter. Once such settings are determined, the cancellation filter may be utilized to cancel signals from a microphone output signal of the implantable microphone. In one arrangement, this entails filtering the digital output of the hearing instrument to generate a filtered output and combining this filtered output the microphone output signal to generate a net signal. This net signal may then be utilized (e.g., input) into signal processing componentry of the implantable hearing instrument for use in generating a subsequent output signal.

As may be appreciated, the cancellation filter may include a transfer function as determined between the output signal of the hearing instrument and the output signal of the microphone. Further, it will be appreciated that multiple such transfer functions may be identified for multiple operating conditions. Accordingly, upon determining the current operating condition by the observer, the utility may select an appropriate transfer function and/or interpolate between one or more transfer functions. Stated otherwise, filter coefficients for the cancellation filter may be selected based on the current operating conditions.

Use of an observer to identify current operating conditions may include the use of an observer that is operative to directly measure a current operating condition. In this regard, the observer may form a sensor that is adapted to directly measure one or more operating conditions/environments associated with the implantable system. In another arrangement, the observer is adapted to deduce parameters representative of current operating conditions. That is, in some instances the current operating condition may not be directly observable. In this latter regard, the observer may utilize another cancellation filter that utilizes a motion sensor to cancel undesired signals from the output signal of an implanted microphone.

In such an arrangement, a motion sensor output may be filtered by an adaptive filter and the resulting filtered motion signal may be cancelled from the microphone output signal. The filter may be adaptive such that by varying the coefficients of the filter, the filtered signal may be altered and, hence, the net signal generated by combining the filtered motion sensor signal with the microphone output signal may likewise be altered. More particularly the filter coefficients may be altered until the energy of the net signal is reduced to a desired degree (e.g., minimized). At such a time, the cancellation may be deemed adequate for the current operating conditions. Accordingly, the settings of the filter that achieves a desired degree of cancellation (i.e., cancellation appropriate for current operating conditions) may be utilized to establish filter coefficients for the filter that filters the output signal of the implantable hearing instrument for cancellation from the microphone output signal.

Unlike cancellation systems that do not use a motion sensor, this approach of setting the filter coefficients of the digital cancellation loop (i.e. cancellation system that removes the filtered digital output from the microphone output signal) based at least in part on settings of the motion sensor filter is relatively insensitive to correlation, so it does not completely cancel out simple sounds (like sirens and beeps). If desired, some portion of the net signal resulting from cancelling the motion sensor signal from the microphone signal can be mixed with the microphone signal cancelled by the filtered digital output signal to give a more natural sound. Such combination may allow for removing some biological noise from the signal input into the implantable hearing system.

In one particular arrangement, the coefficients of the motion sensor filter are all related to a common variable(s), which may include vector variable(s) as well as scalar variable(s). Accordingly, upon reducing the residual energy of the first net signal to the desired degree, a value of the common variable associated with a given operating condition (e.g., posture) may be identified. Accordingly, this value may be utilized in the generation of, for example, filter coefficients or other settings for the digital filter.